Gitlab: install graphviz for doxygen graphs

.gitlab-ci.yml

@@ -23,6 +23,7 @@ generate-docs:
   script:
     # use a setup that provides all dependencies - if the dependencies change,
     # this needs to be updated
+    - yum install -y graphviz
     - source /cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh
     - mkdir build; cd build;
     # the documentation option is off by default, explicitly switch it on